1. Field of the Invention
This invention is in the field of aqueous film coating of pharmaceutical, food, confectionery, and agricultural products, and is specifically concerned with providing coatings from a combination of maltodextrin and cellulosic polymers for coating such things as pharmaceutical tablets, pieces of candy, cereals, and agricultural seeds.
2. Description of the Prior Art
Cellulose polymers such as hydroxypropyl methylcellulose (HPMC) have long been recognized in the art as being suitable for aqueous film coating of pharmaceutical tablets and the like. To those skilled in the art, it is well known that as the molecular weight of the polymer is increased, the tensile strength of the film increases. This is also true for polymer blends whereby a high molecular weight polymer is added to a lower molecular weight polymer producing a substantially stronger film. Rowe, "Some Fundamental Properties of Polymeric Materials and Their Application in Film Coating Formulations", INT. J. PHARM TECH & PROD MFR., (3) Page 5, 1982, states that adding high molecular weight components to a composition of high and low molecular weight grades of a polymer can increase the effective tensile strength of that polymer. Rowe also states on page 3 of the same article that "the data presented almost exclusively pertains to the cellulose derivatives but the trends reported are likely to be the same for all polymers used in film coating."
Although an increase in molecular weight of a polymer blend is expected to increase the tensile strength of the coating, such an increase in molecular weight of the polymer blend decreases the clarity of the coating, and this is a drawback when it is desired to coat a substrate with a clear coating. Accordingly, a trade off has to be made between high tensile strength and high clarity. That is, to reduce cloudiness in a coating, the molecular weight of the polymer blend may be reduced, but this reduction in the molecular weight of the polymer blend would be expected to reduce the tensile strength of the coating.
Maltodextrins, which possess very low average molecular weights, would not be expected to enhance the strength of a film when blended with a polymer of higher molecular weight.
Also, when maltodextrins are combined with a single cellulosic polymer of higher molecular weight, the lower the molecular weight grade of maltodextrin used, the more the tensile strength of the film would be expected to decrease. We have found that, surprisingly, the opposite occurs.